The present invention relates to a method for the laser welding of a composite material to a component in particular for the production of a solar collector element, wherein the composite material comprises a strip-shaped substrate composed of a metal having high reflectivity to laser radiation, said substrate having a first side and a second side, wherein a dielectric coating is situated at least on the first side, and wherein, in order to produce a continuous weld seam or discrete weld spots, a laser beam is projected at an acute orientation angle at least onto the first side of the substrate provided with the dielectric coating.
The invention furthermore relates to a laser-weldable composite material for use in a method of this type, comprising a strip-shaped substrate composed of a metal having high reflectivity to laser radiation, said substrate having a first and a second side, wherein a dielectric coating is situated at least on the first side.
Such a method for laser welding and also laser-weldable composite material are known from the European patent EP 1 217 315 B1 and have proved to be worthwhile in practice. In this case, EP 1 217 315 B1 describes a solar collector element produced in particular on the basis of coated aluminum strip as substrate, wherein, as component, a laser-welded tube for a heat-transfer liquid is fixed thereon. In detail, EP 1 217 315 B1 describes an absorber part connected to the tube on a first side. The absorber part is composed of the composite material comprising the metallic substrate and an optically active multilayer system situated on a second side of the substrate.
As is known, on that side of the strip-shaped substrate which is to be welded and is remote from the optical multilayer system, it is possible optionally to apply, as dielectric layer, a layer which is composed of anodically oxidized or electrolytically brightened and anodically oxided aluminum and can be produced wet-chemically. In this case, the pores of the aluminum oxide layer can be closed off to the greatest possible extent by hot-sealing in the last phase of the wet-chemical process sequence, thus resulting in a surface with long-term stability. The dielectric layer is intended to form mechanical and corrosion-inhibiting protection for the substrate. The connection between the absorber part and the tube, which is composed of copper, in particular, is realized by means of a laser welding method, in particular in an embodiment as a pulse welding method. Laser welding is a fusion welding method, that is to say that the parts to be connected are melted under the action of the laser radiation. A particular feature here is the high power density and, when using pulse welding, the heating and rapid cooling associated with the short duration of action.
If the laser welding is carried out without an additive, the crucial material-to-material bond between the two parts to be connected is composed only of the respective materials of the absorber part and of the tube, wherein, on account of the lower melting point of aluminum, drop-shaped solidified small molten balls are formed on the absorber part, said small molten balls predominantly being composed of the material of the substrate and the material of the coating situated on the side to be welded. The small molten balls bring about the bridging of any gap or air cushion that may be present between the absorber part and the tube.
In particular, the tube and the absorber part can in this case be connected along their abutment joint by means of weld seams which run on both sides of the tube and are formed from weld spots that are spaced apart from one another and are in particular arranged regularly. In order to produce these weld seams, the laser beam is directed into the interstice formed between the tube wall and the surface of the absorber part, the focus lying on the absorber part. In this case, the distance between the focus on the absorber part and the point of contact of the tube on the absorber part is chosen such that the welding post point to be built up from the material of the absorber part can overcome the distance to the tube surface without a hole arising in the absorber part. The laser beam for the production of the weld seam has to be at an acute orientation angle with respect to the surface of the absorber part.
Laser welding with angle relationships of this type is also known from U.S. Pat. No. 6,300,591 B1. That document describes a laser welding process used to connect a planar metallic—but in contrast to EP 1 217 315 B1 uncoated—surface to a cylindrical metallic surface, in particular to that of a tube. For the acute contact angle of the tube wall relative to the planar surface, the latter preferably—as also in EP 1 217 315 B1 —being formed on an absorber part, a value resulting on account of the geometry of the tube, in particular a value resulting in a manner dependent on the diameter-dictated curvature of said tube, for the contact angle and, associated therewith, also for the orientation angle of the laser beam of in each case less than approximately 45° is mentioned as particularly preferred in said document. It is explained that in the case of such angle values, a wedge is naturally formed, which purportedly brings about a focusing of the radiation and hence a maximum energy input into the welding location, though that is not localized. The focusing is purportedly effected as a so-called “non-image concentration”, that is to say without image-generating means, such as lenses, only on the basis of multiple reflection of the radiation at the wedge walls. For this purpose, the laser beam has to be applied to the planar surface and the cylindrical surface simultaneously, and the bodies to be welded, according to the document, have to have a high reflectivity. In particular, copper and aluminum and alloys thereof are claimed as suitable materials in this regard.
According to U.S. Pat. No. 6,300,591 B1, the phenomenon of radiation concentration is purportedly based on the so-called Mendenhall wedge effect, which was described in 1911by Charles Elwood Mendenhall (1872-1935)—Mendenhall, C. E.: “On the emissive power of wedge shaped cavities and their use in temperature measurements”, The Astrophysical Journal 33 (2), pp. 91-97.
However, in the case of a convex surface as described for one of the components to be welded in U.S. Pat. No. 6,300,591 B1, it should be assumed that a divergent beam bundle will arise from a laser beam with light directed in a parallel manner. Moreover, since a high temperature is intended to be generated for welding by the laser beam at the welding location, the use of highly reflective surfaces proves to be counterproductive with regard to the energy input brought about by the laser beam. When different materials are used, it is predominantly the material having the lower melting point that melts in the case of low conductivity. Furthermore, the Mendenhall effect, which is based on multiple reflections, results in an energy input distributed over the reflection locations. Since the proportion of the absorbed energy—assuming transmission of the beam through the material of zero—results from the difference between one and the reflected energy proportion of the beam, the absorbed energy density is significantly lower than when the laser beam is focused by means of a lens. Therefore, it cannot be recognized how a stable weld with good heat transfer is supposed to arise in accordance with U.S. Pat. No. 6,300,591 B1.
Such an orientation of the laser beam requires a very precise and therefore complex alignment and cannot be realized particularly when a plurality of closely adjacent cylindrical components having a relatively large diameter are intended to be welded to the planar component. U.S. Pat. No. 5,760,365 is also concerned with the abovementioned lensless concentration of the laser radiation in a wedge-shaped narrow gap. In said document, metals having a reflectivity to the laser radiation of more than 90 percent are cited as highly reflective metals that should necessarily be used.
DE 38 27 297 A1 also relates to improving the efficiency of the inputting of the energy when joining workpieces by means of laser radiation. Said document relates to an apparatus serving for the laser welding of workpieces, wherein, as in the two patent specifications cited in the introduction, at least one of the workpieces is preferably convexly bent in the manner of a tube at the joining location. In that case, the laser radiation is radiated in a manner directed substantially parallel to a joining gap plane and substantially perpendicular to a joining line and is predominantly polarized in a single plane. In order that the available radiation power can be utilized more efficiently for joining the workpieces, namely in a manner coordinated with the materials thereof, a laser radiation that is polarized perpendicular to the joining gap plane is used. For the predetermined joining geometry, this leads to an optimum energy absorption, namely with grazing incidence of the laser radiation into the joining gap between the workpiece surfaces facing one another. In one configuration of the technical solution described, the laser beam is at least predominantly directed onto only one of the workpieces to be joined. Accordingly, the energy transported by the laser beam, with application of the laser radiation polarized perpendicular to the gap plane, is also coupled into only one of the two workpieces, namely preferably into the one which has the higher melting point and therefore requires more energy for melting. By way of example, materials mentioned include aluminum having a melting point of 600° C. and steel having a melting point of 1600° C. Coatings are not provided. By means of the known apparatus and the corresponding method, the joining of workpieces composed of different materials is thus facilitated, but this presupposes a polarized laser radiation. In order to form the laser beam in striped fashion, optical lenses and/or mirrors are used.
U.S. Pat. No. 4,023,005, which is likewise concerned with the laser welding of components composed of materials that are highly reflective to laser radiation, provides for covering said components with a cladding of low reflectivity metals having a thickness of at least 12.5 μm (0.0005 inch). In particular, nickel layers on copper and also palladium layers on silver or gold are mentioned therein. The metallic coating materials mentioned are in part expensive or the coating process also in part constitutes an increased outlay.